Hi Group,
Sometimes I need to put a letter in front of a number as a fingering.
Is there any possibility to declare "m" (for ex;) as a number so that I
could code<a-m1> as a fingering ?

Do you mean for right hand (stroke) fingering? The following enables
you to use -\A etc for strokefingering. The additional
my-stroke-finger function isn't needed for this but gives better
alignment of the characters when you have a succession of them:
\version "2.15.32"
% shortcuts for stroke finger indications
% can't use a or p, so use upper case for all
P = #(define-music-function (parser location) ()
(apply make-music
(append
(list
'StrokeFingerEvent
'origin location)
(list 'digit 1))))

What about
P=-\rightHandFinger 1
Seems a bit simpler.

Sure is. Probably better to use P=\rightHandFinger #1, and can then use
- or ^ or _ as needed.